1. Field of the Invention
The present invention pertains to methods and apparatus for determining the amount of liquid in a container. More particularly, the present invention is related to techniques for gauging liquid quantities in containers in low or zero net gravity environments, such as in the case of satellites or other space vehicles.
2. Description of the Prior Art
The determination of the amount of liquid in a container in which both gas and liquid are present poses a considerable problem in zero, or near-zero, net gravity environments such as encountered in earth orbit or other space travel. For example, it may be necessary to gauge the amount of liquid fuel in a satellite or space station fuel tank. Typically, a tank for storing liquid that is removed therefrom as needed also contains a pressurant gas for maintaining the liquid under appropriate pressure to permit the liquid to be tapped off of the tank. However, due to the lack of gravitational forces, the liquid and gas are generally completely mixed throughout the volume of the tank, absent the use of any techniques to separate the two and gather the liquid toward one region of the tank. Consequently, the gauging of the amount of liquid in the container is not readily carried out.
Several techniques are known for attempting to gauge liquid in a container under such circumstances. For example, the container may be accelerated to settle the liquid to a specific location in the container, then the level of the liquid at that location may be determined with a level detector, or liquid/gas interface sensor. However, so accelerating the container requires a complicated propulsive system to produce the accelerating force, which also affects the entire system of which the container is a part. Another technique utilizes the container, or tank, in a blowdown mode. After the tank containing liquid is pressured with the pressurant gas, the tank is isolated from the pressurant supply. Then, as liquid is withdrawn from the tank, the change in pressure within the tank is detected and the related expansion of the pressurant is determined to determine the quantity of liquid remaining within the tank. However, utilizing the tank in a blowdown mode results in a decrease in the liquid withdrawal rate as the pressure in the tank is reduced. Therefore, in order to achieve a desired flow rate at the diminished tank pressure, the initial tank pressure must be increased, resulting in a heavier initial tank weight. Furthermore, any gas leakage from the tank produces an error in the gauging of the liquid remaining. Yet another technique requires use of a dedicated pressurant tank from which gas may be supplied to the liquid container by means of a pressure regulator. The amount of liquid in the tank can thus be determined by gauging the quantity of pressurant gas added to the tank containing the liquid. However, such technique obviously requires a dedicated pressurant tank, and leakage of the pressurant gas may be mistaken for liquid loss. Further, of the aforementioned techniques, only the first technique resulting in settling of the liquid in one region of the container is usable for gauging cryogenic liquids.
It is an object of the present invention to provide a method and apparatus for gauging the liquid in a container under zero, or near-zero, net gravitational force without the need for a special propulsive system, or any acceleration to be imposed on the container or the system of which the container is a part.
It is a further object of the present invention to provide such a technique without the need for a dedicated gas pressurant source, and without the need for an increase in tank working pressure and weight to accommodate operation in a blowdown expulsion mode.